Being familiar with 35mm traditional film and digital photography, this at least for me is a good starting point to wrap my head around what we refer to as “Wide-field Astrophotography.”
In traditional photography, on a full frame 35mm camera, a lens is generally considered to be "wide angle" when its focal length is less than around 35mm. This translates into an angle of view which is greater than about 55° across your photo's widest dimension. The definition of ultra-wide is a little fuzzier, but most agree that this realm begins with focal lengths somewhere around 20-24 mm and less.
Regardless, the key concept is this: the shorter the focal length, the more you will tend to notice the unique effects of a wide angle lens.
The above diagrams depict the maximum angles that light rays can take when hitting your camera's sensor. The location where light rays cross is not necessarily equal to the focal length, but is instead roughly proportional to this distance. The angle of view therefore still increases similarly.
What makes a wide angle lens unique? A common misconception is that wide-angle lenses are primarily used for when you cannot step far enough away from your subject, but yet still want to capture all of this subject in a single camera frame. Unfortunately, if one were to only use it this way they'd really be missing out. In fact, wide angle lenses are often used for just the opposite: when you want to get closer to a subject!
So, let's take a closer look at just what makes a wide angle lens unique:
Its image encompasses a wide angle of view It generally has a close minimum focusing distance
Although the above characteristics might seem pretty basic, they result in a surprising range of possibilities. The rest of this page focuses on techniques for how to best use these traits for maximal impact in wide angle photography.
WIDE ANGLE PERSPECTIVE
Obviously, a wide angle lens is special because it has a wide angle of view — but what does this actually do? A wide angle of view means that both the relative size and distance is exaggerated when comparing near and far objects. This causes nearby objects to appear gigantic, and far away objects to appear unusually tiny and distant. The reason for this is the angle of view:
Wide Angle Lens Telephoto Lens (objects are very different sizes) (objects are similar in size)
Even though the two cylinders above are the same distance apart when photographed with each lens, their relative sizes are very different when one fills the frame with the closest cylinder. With a wider angle of view, further objects therefore comprise a much lower fraction of the total angle of view.
A misconception is that a wide angle lens affects perspective, but strictly speaking, this isn't true. Perspective is only influenced by where you are located when you take a photograph. However, in practical use, wide-angle lenses often cause you to move much closer to your subject — which does affect perspective.
WIDE ANGLE PERSPECTIVE vs. TELEPHOTO
A lens is generally considered to be "medium telephoto" when its focal length is greater than around 70 mm. However, many don't consider a lens a "full telephoto" lens until its focal length becomes greater than around 135 mm. This translates into an angle of view which is less than about 15° across your photo's widest dimension.
A common misconception is that telephoto lenses are just for capturing distant objects. While this is a legitimate use, there's a whole array of other possibilities, and often time’s distant objects are better photographed by simply getting a little closer.
Yes, this isn't practical with a lion, or a deep sky faint-fuzzy object but a pet or a person will likely appear better when they aren't photographed from afar. Why? The distance from your subject actually changes your photo's perspective, even if your subject is still captured at the same size in your camera frame.
A narrow angle of view means that both the relative size and distance is normalized when comparing near and far objects. This causes nearby objects to appear similar in size compared to far away objects — even if the closer object would actually appear larger in person. The reason for this is the angle of view.
So what does that mean?
A telephoto lens not only magnifies the subject, but it also tends to compress perspective and provides a sufficiently narrow angle of view to Isolate the subject.
The perspective compression characteristic of a telephoto lens makes distant background objects appear closer to your subject and in a more similar scale. Simply put, a telephoto lens "brings subjects closer", while a wide angle lens seems to make subjects appear further away than they actually are. The result is a compressed and expanded perspective, respectively.”
This focal length comparison animation located at Canon USA’s website helps to demonstrate how the focal length of a lens affects angle of view:
(http://www.usa.canon.com/app/html/EFLenses101/focal_length.html )
So what does all this have to do with astrophotography?
If you recall, some of you rather than invest in complicated and expensive gear got your start in astrophotography via 35mm film of a DSLR and the lenses which were often used for this purpose fell within the 28 – 300mm focal length range.
However, quality optics of your typical fixed focal length prime lenses in the 300-800mm range, carry hefty price tags starting around $1200 for a f/4.0 300mm to more than $13K; the other issue related to DSLR astrophotography are HEAT and NOISE. These considerations do not preclude one from engaging solely in DSLR astrophotography.
ENTER THE TELESCOPE, ASTROGRAPHS AND THE LIKE
OK, so we’ve come full-circle – sort of.
When one is considering Wide-field astrophotography with CCD cameras, the level of complexity therein increases. This brings us back to telescope optics and pixel size.
The best source of information IMHO related to this topic which attempts to simplify the concepts of Wide-field astrophotography with CCD cameras, is Apogee Imaging Systems CCD University: http://www.ccd.com/ccdu.html
While this is certainly not an exhaustive source, it does help to put into perspective what we tend to discuss as Wide-field astrophotography in its simplest form. The complexity at least to me does not reside with DSLR based astrophotography. It rears its ugly head when one introduces CCD cameras and the major issues one is confronted when attempting to match a particular scopes focal length and optics, choice of imaging planetary or deep space objects and ones seeing conditions for any given site when choosing a CCD camera.